Apparently is up to individual players, according to TheProCap.com website. Here is one quote from there:
"As the first NFL player to wear one, I took a lot of ribbing from my teammates. 'Bubblehead' or 'Kazoo', as I was referred to, soon, learned that I could take and make hits without concussive side effects. Prior to ProCap I had numerous concussions. The ProCap extended my pro career by 5 years, including four Super Bowls,and I have a clear head in my life after football," said Mark Kelso (Buffalo Bills All-Pro Safety, 1985-1993).

You may be onto something there. That could definitely be tested too by comparing the outputs from the helmet-mounted and in-ear accelerometers for a soft outer shell. However, I suspect the answer is durability, since a soft outer shell might get damaged more easily.

I have a question. Why not put energy absorbing material on the outside of the helmet? It would greatly reduce energy transfer during helmet hits while still allowing full movement and vision. Or am I missing something. I played football in high school and it just occurred to me that there was no reason for the helmet to have a hard outer surface.

The NFL has been talking to the IndyCar guys about adapting their design for an in-ear accelerometer that would go inside the football players ear, like ear plugs, instead on on the helmets. (You can't get accurate head trauma measurements by putting it on the helmet, says IndyCar, because the padding inside the helmet is designed not to transfer shock):
http://www.eetimes.com/electronics-news/4373711/IndyCar-safety-harnesses-MEMS#
IndyCar says NFL rejected their current earpiece design for use in football, because it was too big and power hungry. But within a couple months, IndyCar plans to have a much smaller earpiece designed using ADI's single-chip three-axis accelerometer discussed above, which will be small enough for NFL plus can be battery powered since it uses 15-times less power. For testing helmets, it will also be useful to put a high-G accelerometer on both the helmet and in the ear canal, then you can evaluate different padding arrangements in the helmet and directly measure which is better by comparing the accelerometer outputs from the helmet-mounted and in-ear mounted accelerometers.